I'm not saying the Yankees' bullpen is not good, because it's great, but there are a few question marks, and you don't want to get caught off guard if players like Edwar Ramirez or Jose Veras don't perform up to expectations. Now, the Yankees have a great oppurtunity, most every Yankee fan who hears what i have to say here, will disagree.
If you want a definite World Series winning team, you may need Joba Chamberlain in the bullpen. The Yankees have a few guys who are capable of filling in the role of the 5th starter, i.e Phil Hughes, Alfredo Aceves, Ian Kennedy and Phil Coke.
Joba has nasty stuff, he could be a great number 1 guy within the next few years, but he has a history of shoulder problems that may say otherwise. We Yankee fans want to believe Joba's shoulder wont cave in, but he could be just another shoulder injury away from becoming the next Mark Prior, Kerry Wood etc...
We saw last year that Joba fit into the starting role very well, perhaps with the propper conditioning for the job, we won't see him suffer anymore freak injuries.
Again, i'm NOT saying put Joba in the bullpen. What i am saying is the Yankees do have the option to put him in the bullpen and succeed just as well, maybe even more than had they kept him as a starter.
The main risk with having Joba in the bullpen, would take away from him improving as a starter, and maybe eliminate starting from his future entirely.
The Yankees may have the bullpen with the greatest potential in the majors, Brian Bruney, Jose Veras, Edwar Ramirez and Mark Melancon each have nasty stuff, but we've only seen how succesfull they can be briefly. Putting Joba in the 8th inning role would give the Yankees the best bullpen in the majors, with a top 3 starting rotation. Ofcourse their lineup, when healthy... speaks for itself.
Even with the whole A-Rod, Joe Torre and DUI situations as possible distractions for the players, i still see the Yankees having one of their best seasons since the beginning of the new century.

